Superheterodyne receiver



Aug 1935- w. VAN B. ROBERTS 2,050,807-

"SUPERHETERODYNE RECEIVER Original Filed June 2, 1932 INVENTOR WALTERPatented Aug. 11, 1936 UNITED STTES.

SUPERHETERODYNE RECEIVER Walter van B. Roberts,

Princeton, N. J., assignor to Radio Corporation of America, a.corporation of Delaware Application June 2, 1932, Serial No. 614,898Renewed June 20, 1934 6 Claims.

My present invention relates to heterodyne systems, and moreparticularly to a superheterodyne receiver including a novel type offrequency changer.

A modern type of multi-electrode electron discharge tube which is knownas a radio frequency pentode tube utilizing a suppressor grid, lends itsself in an effective manner to utilization in the frequency changercircuit of a superheterodyne receiver. This type of suppressor grid tubebriefly comprises the usual cathode, control electrode and anode,- apositive screen grid being disposed between'the control grid and theanode for shielding the anode, the suppressor grid consisting of a griddisposed between the screen grid and the anode, and usually surroundingthe screen grid, the suppressor grid being maintained atnegative'potentials with respect to cathode potential.

Now, I have discovered that the suppressor grid of this type of pentodetube can be readily used, in the frequency changer circuit of asuperheterodyne receiver, as a control electrode of an electronic systemincluding the screen grid, functioning simultaneously as a source ofelectrons, and the anode.

- Accordingly, it may be stated that it is one of the main objects of mypresent invention to provide a compositedetector-oscillator for aheterodyne receiving system, the composite detectoroscillator, orfrequency changer, including an electron discharge tube having itscathode and control electrode coupled to a source of signal energy, andhaving a suppressor grid disposed between a positive screen grid and ananode, the suppressor grid being arranged to have local osoillationsimpressed thereon, and the control electrode functioning to modulate theaction of the suppressor grid and the anode, while the screen grid notonly'functions as an electrostatic shield between the suppressor gridand anode and the input electrodes, but also as a virtual cathode withrespect to the suppressor grid and anode.

Another important object of the present invention is. to provide afrequency changer circuit for a superheterodyne receiver, which circuitcomprises but a single tube upon whose input electrodes isimpresseddesired modulated signal energy, the anode of the tube beingregeneratively coupled with an outer grid which is operated at negativepotentials with respect to cathode potential, a middle grid, beingdisposed between the control electrode and the outer grid, functioningas'a screen. electrode as well as a. virtual cathode for the outer gridand the anode.

Another object of the present invention is to provide a radio receiverof the superheterodyne type which comprises an intermediate frequencyamplifier having its. input coupled to the output circuit of a singlevtube frequency changer, the tube being a radio frequency pentode tubeemploying a grounded suppressor grid, the anode circuit of the tubebeing coupled to the suppressor grid, a source of signal energy beingcoupled between the control electrode and a cathode which is at apositive potential with respect to the control electrode and suppressorgrid.

Still other objects of the present invention are to improve generallythe simplicity and eificiency of frequency changer circuits, and toparticularly provide a superheterodyne receiver which includes acomposite first detector-oscillator which is not only durable andreliable in operation, but economically manufactured and assembled.

The novel features which I believe to be characteristic of myinventionare set forth in particularity in the appended claims, theinvention itself, however, as to both its organization and method ofoperation will best be understood by reference to the followingdescription taken in connection with the drawing in which I haveindicated diagrammatically several circuit organizations whereby myinvention may be carried into effect.

In the drawing,

Fig. 1 diagrammatically shows a circuit embodying the present invention,

Fig. 2 shows another form of the invention,

Fig. 3 shows another embodiment of the invention wherein an externalsource of local oscillations is employed,

Fig. 4 shows still another modification, and

Fig. 5 is still another embodiment of the invention.

Referring now to the accompanying drawing in which like characters ofreference indicate the same circuit elements in the different figures,there is shown diagrammatically in Fig. 1 a conventional type ofsuperheterodyne receiver which comprises the usual grounded antennacircuit A, G, which is coupled, as at M1 to the tunable input circuit ofthe composite detector-oscillator which forms the subject matter of thepresent invention.

The composite detector-oscillator includes a tube l which is known as aradio frequency pentode tube and employs a suppressor grid 2. Thissuppressor, or outer, grid is'grounded, as at 3, and includes in seriestherewith a tunable antiresonant oscillation circuit comprising aninductance coil 4 and a variable tuning condenser 5. The anode of tube Iis connected to the positive terminal of a source of current B throughan inductance coil 6, inductively. coupled to the coil 4, and ananti-resonant oscillation circuit comprising an inductance coil 1 and afixed condenser 8.

The circuit 1, 8 is maintained fixedly resonant to the desiredintermediate frequency, as is well known to those skilled in the art,and this circuit is coupled, as at M2, to a second oscillation circuit 9which is connected in the input of the 60 usual intermediate frequencyamplifier. The output of the intermediate frequency amplifier is shownconnected to a second detector, an audio frequency amplifier and to anytype of utilization means, such as a loud speaker, head phones, or thelike. It is to be clearly understood that these elements following thecomposite detector-oscillator tube I are conventional in nature, and arewell known to those skilled in the art, the present invention residingin the tube I and its associated circuits.

The control grid of tube I is connected to the high potential side ofthe tunable input circuit II! of tube I, while the cathode is connectedto the low potential side thereof. The variable tuning condenser I l isarranged to be mechanically operated with the condenser 5, and anyarrangement well known to those skilled, in the art may be employed forthis purpose, it being pointed out that the dotted lines I2 designate amechanical uni-control device for operating the rotors of condensers IIand 5 in such a manner that a constant difference frequency ismaintained between the tuned circuits II! and 5, 4 throughout the tuningrange of the receiver.

0ne method of maintaining such a constant difference frequency betweenthe two circuits is disclosed by W. L. Carlson in U. S. Patent 1,740,331of December 17, 1929.

It will be understood, of course, that the circuit 5, 4 is tuned to thedifferent local oscillation frequencies desired for the heterodyningpurpose. The cathode circuit includes the usual grounded control gridbias resistor R, shunted by a radio frequency by-pass condenser C. Inorder to electrostatically shield the tuned circuit 5, 4 and thefeed-back coil 6, a grounded metallic screen I 3 may be disposed aroundthese last mentioned circuit elements.

In considering the functioning and operation of the arrangement shown inFig. 1, it should be understood that, in general, the grid nearest theanode, that is the grid 2, may be operated at low, or negative,potentials, and thus functions in the manner of a control grid. Theinner grid, that is the usual'control grid, then acts to modulate theaction of the outer grid 2 and anode, while the middle grid, or screengrid, acts as an electrostatic screen as well as a virtual cathode forthe outer electrodes, that is the suppressor grid 2 and the anode. It isnot believed necessary to go into the details'of the physical andelectrical phenomena which occur in the operation of tube I. It isbelieved sufficient to point out that the functioning of the compositedetector-oscillator circuit is along the following lines:

Any convenient oscillator circuit of the feedback type, such as formedby elements 4, 5 and 6 of Fig. 1, is arranged to produce oscillations incooperation with the grid 2 acting as control grid on the electronstream to plate I, said stream issuing from the screen grid which thusmay be considered as a virtual cathode. The strength of oscillationsthus generated depends upon the strength of the electron stream abovementioned, and this in turn depends upon the potential of the innergrid, that is, the grid next to the actual cathode. Thus the oscillationstrength is varied in accordance with the signal voltage impressed onthe inner grid which results, as is well known in the art, in theproduction of currents in the plate circuit of frequencies equal to thesum and difference of the signal and oscillation frequencies. Either thesum or the difference (but usually the difference) may be used as theintermediate frequency to which circuit 1, 8 is tuned.

In Fig. 2 there is shown a modified form of the invention, the tube Ihaving its input electrodes coupled between opposite sides of the tunedcircuit ID, as m Fig. 1, and the output oscillation circuit I, 8 beingcoupled to the input of the intermediate frequency amplifier in the samemanner as in Fig 1. The arrangement shown in Fig. 2 differs from thatshown in Fig. 1 in that the tunable local oscillation circuit 4, 5 isdisposed in the anode circuit of the tube, whereas in Fig. 1 it isconnected in the suppressor grid circuit. Positive potential is suppliedto the anode through a radio frequency choke l4, a direct currentblocking condenser being connected between the anode and the inductancecoil 1.

In order to provide local oscillations, the coil 4, one side of which isgrounded, is inductively coupled, as at M3, to the coil 4 connected inseries with the suppressor grid 2. It will be noted that the suppressorgrid 2 is maintained at a more negative potential with respect to thecathode than is the inner grid by means of a biasing source I5 havingits negative potential connected to one side of the coil 4', while itspositive terminal is grounded. This arrangement is used because I havefound this additional bias advantageous with some tubes. The operationof this form of invention is substantially identical with that describedin connection with Fig. 1.

Still another form of the invention is shown in Fig. 3, and in this formof the invention the suppressor grid 2 of the tube I has impressed uponit local oscillations from an external local oscillator 20. One side ofthe local source of voltage 20 is connected to the negative terminal ofthe biasing source I5, while the positive terminal of the source I5 isgrounded. The control grid of tube I is shown biased by a negativebiasing source I5 and the source of signal energy is shown connectedbetween the control grid and cathode of tube I, it being conventionallydesignated by the symbol Es. In this form of the invention it is not ofcourse, necessary to couple the anode circuit to the suppressor gridcircuit. However, the action of the circuit including tube I is stillthe same as in the case of Figs. 1 and 2. Here, as in the last mentionedtwo figures, the suppressor grid 2 functions in the manner of a controlgrid, and the usual control grid functions to modulate the action of thesuppressor grid, the screen grid again operating as a virtual cathodefor the anode and outer grid 2, while also acting as an electrostaticscreen.

In Fig. 4 is shown. another modification wherein the source of anodepotential B has its negative terminal connected to one side of thetunable local oscillation circuit 4, 5, the coil 4 being inductivelycoupled with the inductance coil 4' connected in series between thesuppressor grid 2 and the negative terminal of the biasing source I5.The positive terminal of the source I5 is connected to the cathode sideof tube I. It will be observed that this arrangement is quite similar tothat shown in Fig. 2 since the tunable oscillation circuit is disposedin the anode circuit of tube I, while the suppressor grid is maintainedat a more negative potential than the cathode of tube I.

The difference between this arrangement and that shown in Fig. 2 residesin the fact that coil 1 of the intermediate frequency tuned circuit isof sufficient inductance, usually, to allow it to act as a choke for theoscillation frequency while the condenser 8 acts as the blockingcondenser which prevents the plate direct current potential fromreaching the tuning condenser 5 and coil 4. Where the intermediatefrequency is sufliciently low to permit this arrangement it ispreferable because it does not require an additional choke and blockingcondenser. It will be obvious to one skilled in the art that theoscillator tuning condenser 5 may be equally well connected across coil4', or between the live ends of coils 4 and 4', in Figs. 2 and 4.

In Fig. 5 is shown a form of the invention wherein, as shown in Fig. 1,the tunable oscillation circuit 5, 4 is disposed in series with thesuppressor grid, one side of the circuit being connected to the negativeterminal of the suppressor grid biasing source I5. The anode circuit oftube I includes a feed-back coil 6 which is inductively coupled with thecoil 4, and it will be noted that the negative terminal of the source Bis grounded in common with the positive terminal of source l5. Thedifierence between this circuit and that shown in Fig. 1 resides in thefact that the intermediate frequency circuit and coil 6 are interchangedin series position, and extra bias is shown on the suppressor grid.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

1. An oscillator-converter comprising a tube provided with a cathode, asignal grid, an anode, a local oscillation grid, and an auxiliary gridbetween said signal and local oscillation grids, a tunable signalcircuit between the signal grid and cathode, a tunable local oscillatorcircuit electrically associated with said oscillation grid, anintermediate frequency circuit in said anode circuit, means formaintaining said oscillation grid negative with respect to said cathode,said auxiliary grid acting as an electrostatic screen between saidsignal and oscillation grids, and as a virtual cathode, said oscillatorcircuit being connected between the anode and cathode and being includedin said intermediate frequency circuit.

2. An oscillator-converter comprising a tube provided with a cathode, asignal grid, an anode, a local oscillation grid, and an auxiliary gridbetween said signal and local oscillation grids, a tunable signalcircuit between the signal grid and cathode, a tunable local oscillatorcircuit electrically associated with said oscillation grid, anintermediate frequency circuit in said anode circuit, means formaintaining said oscillation grid negative with respect to said cathode,said auxiliary grid acting as an electrostatic screen between saidsignal and oscillation grids, and as a virtual cathode, saidintermediate frequency circuit including a condenser and a coil, saidlocal oscillator circuit being arranged in series with said coil andcondenser.

3. Means for producing oscillation including a plate, grid and electronstream source, means for maintaining said grid substantially morenegative than said source, means for controlling the strength of saidoscillations in accordance with a signal, comprising a second grid, thefirst grid being maintained at a substantially more negative potentialthan said second grid, and the latter being it a substantial negativebias with respect to said source, and means for shielding one grid fromthe other comprising a third grid interposed between them, andutilization means associated with said anode for utilizing currents offrequency equal to the difference of the oscillation and signalfrequencies, said utilization means including a coil shunted by acondenser, and a tunable local oscillator circuit connected in seriesbetween the cathode and said utilization means.

4. In a modulated carrier wave receiver, a mixer network comprising anelectron discharge tube of the type including at least an electronemitter, a positive output anode, a control grid, a resonant wave inputcircuit connected between the emitter and grid, an oscillation circuitconnected between the anode and emitter, said oscillation circuit beingtuned to a frequency difiering from the wave frequency, a secondoscillation circuit, tuned to the difference frequency, connectedbetween the anode and emitter, a cold electrode disposed within the tubeto affect the electron stream thereof, means reactively coupling thecold electrode to the anode, means for maintaining said cold electrodeand control grid at negative direct potentials with respect to theemitter, and a positive cold electrode disposed between the control gridand cold electrode.

5. In a modulated carrier wave receiver, a mixer network comprising anelectron discharge tube of the type including at least an electronemitter, a positive output anode, a control grid,

a resonant wave input circuit connected between the emitter and grid, anoscillation circuit con nected between the anode and emitter, saidoscillation circuit being tuned to a frequency differing from the wavefrequency, a second oscillation circuit, tuned to the differencefrequency, connected between the anode and emitter, a cold electrodedisposed within the tube to affect the electron stream thereof, meansreactively coupling the cold electrode to the anode, means formaintaining said cold electrode and control grid at negative directpotentials with respect to the emitter, and a positive cold electrodedisposed between the control grid and cold electrode, said positiveelectrode being at the same alternating potential as the emitter wherebyit acts as an electrostatic screen.

6. In a modulated carrier wave receiver, a mixer network comprising anelectron discharge tube of the type including at least an electronemitter, a positive output anode, a control grid, a resonant wave inputcircuit connected between the emitter and grid, an oscillation circuitconnected between the anode and emitter, said oscillation circuit beingtuned to a frequency differing from the wave frequency, a secondoscillation circuit, tuned to the difference frequency, connectedbetween the anode and emitter, a cold electrode disposed within the tubeto aifect the electron stream thereof, means reactively coupling thecold electrode to the anode, means for maintaining said cold electrodeand control grid at negative direct potentials with respect to theemitter, and a positive cold electrode disposed between the control gridand cold electrode, said negative cold electrode being disposed betweenthe anode and the positive cold electrode.

WALTER VAN B. ROBERTS.

